In comparison with microwave point-to-point radio relays, optical connections for data transmission between satellites in space have been shown to be very advantageous. Thanks to the extremely short wavelength of light, an optical beam can be radiated very easily by means of a relatively small optical device at a narrow space angle. By means of the antenna gain achieved in this way, a high data rate can be transmitted with low transmission output. Corresponding directional antennas for microwave connections are comparatively heavy and require a relatively large space. However, because an optical transmitted beam can be easily collimated, it requires an extremely exact determination and tracking of the direction of the transmitted beam as well as that of the reception direction.
It must be considered an additional difficulty that a satellite, being a body which moves dissociated in space, cannot bleed off mechanical vibrations via fixed connections or a surrounding atmosphere and therefore displays mechanical self-resonance which still occurs in the range of some kilohertz and can be detected because of the vibrations induced by the rocket engine, in particular following orbit changes or correcting maneuvers. These mechanical vibrations are transferred to an optical data transmission device on board the satellite and impair the correct alignment of the transmitted beam as well as the maintenance of the reception direction.
A further problem caused by the transmitted beam being radiated through only a narrow angular range is the establishment of an optical connection between two satellites, since both optical transmission devices must perform their extremely mutual alignment on their own. In tests and concepts for solving these problems made up to now, contact establishment was divided into three sections. First a mutual acquisition phase takes place, thereafter the respective reception direction and the respective transmitted beam are exactly aligned in respect to each other, and then the alignment is exactly tracked. Furthermore, the devices used for the individual steps are usually sectionalized.
In a first step, the optical transmitting and receiving devices are set by means of servo motors to a required value at a large angular range with comparatively little precision. Fine adjustment is usually provided by means of a small, low-mass piezo-electrically adjustable mirror, by means of which the effect of mechanical vibrations of the satellite body is also compensated. Finally, the direction of the transmitted beam must be oriented slightly differently than the reception direction, if both satellites move in respect to each other.
The light must be transmitted at a defined lead correction angle to the counter station in order to impact on the satellite. Therefore this lead correction angle is approximately determined from twice the running time of the light between the two satellites and from their relative velocity in respect to each other. To make mutual acquisition possible, a considerably more powerful transmitter is provided in a conventional optical transmission device, which radiates through a larger spatial angle than the transmitted beam provided for the actual communication and which is paralactically mounted in respect to the optical device of the actual communication system (T.T. Nielsen "Pointing, Acquisition and Tracking System for the Free Space Laser Communication System SILEX", SPIE, vol. 2381, "Free-Space Laser Communications Technologies VII", pp. 194 to 205, ISBN 0-8194-1728-9).
Based on already existing data regarding the position of the satellite intended as the counter station, the device on a satellite starts to illuminate a defined angular range by means of the considerably stronger optical transmitter identified as a beacon, while a corresponding angular range is scanned on board the other satellite as the reception direction. As soon as the beacon signal has been detected, the receiving direction is set exactly and, on the basis of its angular change, the lead correction angle for the transmitted beam is determined and the latter is transmitted to the other satellite. After its reception, the other satellite will set the reception direction exactly, will also return a transmitted beam with the matched lead correction angle and shut down the operation of the beacon. Finally, the exact tracking of the transmission and reception direction is performed, wherein the lead correction angle is separately readjusted on the basis of the angular velocity of the respective counter station.
The coarse setting of the transmission and reception direction takes place by rotating the telescope provided for this around two axes by means of reduction-geared stepper motors. Fine adjustment is performed by a mirror immediately behind the telescope. The former can be tilted around two axes, wherein the tilting movement takes place by coils located in permanent magnetic fields and connected with the mirror. The position of the mirror is detected by inductive sensors.
The received light beam aligned in this way is distributed to the sensors required for the individual stages of the connection establishment and for maintaining the connection. In the present exemplary embodiment these are two separate CCD sensors similar to those which can also be found in video cameras. The CCD sensor used for acquisition has a resolution of 288.times.288 pixels and therefore a relatively wide field of view. It is the job of this sensor to detect the pixel which in comparison is illuminated the strongest in order to monitor its placement into the range of the much narrower field of view of the CCD sensor provided for controlling the precise alignment and tracking. The CCD sensor provided for tracking only has 14.times.14 pixels in order to make possible its rapid read-out, since the data obtained by means of this sensor are also used for compensating the self-resonance of the satellite. Finally, the received beam is supposed to be distributed over many central pixels of the sensor, whose respective illumination is compared, because of which the resolution of the angle falls beneath the threshold generated by the size of the individual pixels. Some pixels adjoining these four pixels are also read out for estimating the dark current and correcting errors as a result thereof. Therefore the achievement of the stable final state requires several steps:
First, detection by the acquisition sensor, then the transfer into the field of view of the sensor intended for tracking the received beam, furthermore the alignment on the four central pixels within its detected range and finally the most exact possible alignment by means of the quantitative comparison of the photo flows delivered by them.
The disadvantages of the prior art outlined by means of this example are, for one, the considerable complexity of the entire devices required for acquisition, alignment and tracking of the transmitted beam and the received beam.
In addition, no optical communications signal has as yet been detected during the exact alignment of the transmitted beam and the received beam, a high-speed photodiode with a front end downstream thereof is required for this. CCD sensors in particular require complex electronics for their control, which require space and add weight, this all the more because under space conditions the complexity of electronic components results in an increased failure probability, just consider the damaging effects of gamma radiation, and therefore requires the availability of redundant components.